DESCRIPTION

In this chapter of the tutorial, we will create a very basic Catalyst web application, demonstrating a number of powerful capabilities, such as:

Helper Scripts

Catalyst helper scripts that can be used to rapidly bootstrap the skeletal structure of an application.

MVC

Model/View/Controller (MVC) provides an architecture that facilitates a clean "separation of control" between the different portions of your application. Given that many other documents cover this subject in detail, MVC will not be discussed in depth here (for an excellent introduction to MVC and general Catalyst concepts, please see Catalyst::Manual::About). In short:

Model

The model usually represents a data store. In most applications, the model equates to the objects that are created from and saved to your SQL database.

View

The view takes model objects and renders them into something for the end user to look at. Normally this involves a template-generation tool that creates HTML for the user's web browser, but it could easily be code that generates other forms such as PDF documents, e-mails, spreadsheets, or even "behind the scenes" formats such as XML and JSON.

Controller

As suggested by its name, the controller takes user requests and routes them to the necessary model and view.

ORM

The use of Object-Relational Mapping (ORM) technology for database access. Specifically, ORM provides an automated and standardized means to persist and restore objects to/from a relational database and will automatically create our Catalyst model for use with a database.

CREATE A CATALYST PROJECT

Catalyst provides a number of helper scripts that can be used to quickly flesh out the basic structure of your application. All Catalyst projects begin with the catalyst.pl helper (see Catalyst::Helper for more information on helpers). Also note that as of Catalyst 5.7000, you will not have the helper scripts unless you install both Catalyst::Runtime and Catalyst::Devel.

In this first chapter of the tutorial, use the Catalyst catalyst.pl script to initialize the framework for an application called Hello:

$ catalyst.pl Hello
created "Hello"
created "Hello/script"
created "Hello/lib"
created "Hello/root"
...
created "Hello/script/hello_create.pl"
Change to application directory and Run "perl Makefile.PL" to make sure your install is complete
$ cd Hello

Note: If you are using Strawberry Perl on Win32, drop the ".pl" from the end of the "catalyst.pl" command and simply use "catalyst Hello".

The catalyst.pl helper script will display the names of the directories and files it creates:

Catalyst will "auto-discover" modules in the Controller, Model, and View directories. When you use the hello_create.pl script it will create Perl module scaffolds in those directories, plus test files in the "t" directory. The default location for templates is in the "root" directory. The scripts in the script directory will always start with the lowercased version of your application name. If your app is MaiTai, then the create script would be "maitai_create.pl".

Though it's too early for any significant celebration, we already have a functioning application. We can use the Catalyst supplied script to start up a development server and view the default Catalyst page in your browser. All scripts in the script directory should be run from the base directory of your application, so change to the Hello directory.

Run the following command to start up the built-in development web server (make sure you didn't forget the "cd Hello" from the previous step):

Note: The "-r" argument enables reloading on code changes so you don't have to stop and start the server when you update code. See perldoc script/hello_server.pl or script/hello_server.pl --help for additional options you might find helpful. Most of the rest of the tutorial will assume that you are using "-r" when you start the development server, but feel free to manually start and stop it (use Ctrl-C to breakout of the dev server) if you prefer.

Point your web browser to http://localhost:3000 (substituting a different hostname or IP address as appropriate) and you should be greeted by the Catalyst welcome screen (if you get some other welcome screen or an "Index" screen, you probably forgot to specify port 3000 in your URL). Information similar to the following should be appended to the logging output of the development server:

Note: Press Ctrl-C to break out of the development server if necessary.

HELLO WORLD

The Simplest Way

The Root.pm controller is a place to put global actions that usually execute on the root URL. Open the lib/Hello/Controller/Root.pm file in your editor. You will see the "index" subroutine, which is responsible for displaying the welcome screen that you just saw in your browser.

Later on you'll want to change that to something more reasonable, such as a "404" message or a redirect, but for now just leave it alone.

The "$c" here refers to the Catalyst context, which is used to access the Catalyst application. In addition to many other things, the Catalyst context provides access to "response" and "request" objects. (See Catalyst::Runtime, Catalyst::Response, and Catalyst::Request)

$c->response->body sets the HTTP response (see Catalyst::Response), while $c->welcome_message is a special method that returns the welcome message that you saw in your browser.

The ":Path :Args(0)" after the method name are attributes which determine which URLs will be dispatched to this method. (You might see ":Private" if you are using an older version of Catalyst, but using that with "default" or "index" is currently deprecated. If so, you should also probably upgrade before continuing the tutorial.)

Some MVC frameworks handle dispatching in a central place. Catalyst, by policy, prefers to handle URL dispatching with attributes on controller methods. There is a lot of flexibility in specifying which URLs to match. This particular method will match all URLs, because it doesn't specify the path (nothing comes after "Path"), but will only accept a URL without any args because of the ":Args(0)".

The default is to map URLs to controller names, and because of the way that Perl handles namespaces through package names, it is simple to create hierarchical structures in Catalyst. This means that you can create controllers with deeply nested actions in a clean and logical way. For example, the URL http://hello.com/admin/articles/create maps to the package Hello::Controller::Admin::Articles, and the create method.

While you leave the script/hello_server.pl -r command running the development server in one window (don't forget the "-r" at the end!), open another window and add the following subroutine to your lib/Hello/Controller/Root.pm file:

The development server noticed the change in Hello::Controller::Root and automatically restarted itself.

Go to http://localhost:3000/hello to see "Hello, World!". Also notice that the newly defined 'hello' action is listed under "Loaded Private actions" in the development server debug output.

Hello, World! Using a View and a Template

In the Catalyst world a "View" itself is not a page of XHTML or a template designed to present a page to a browser. Rather, it is the module that determines the type of view -- HTML, PDF, XML, etc. For the thing that generates the content of that view (such as a Template Toolkit template file), the actual templates go under the "root" directory.

To create a TT view, run:

$ script/hello_create.pl view HTML TT

This creates the lib/Hello/View/HTML.pm module, which is a subclass of Catalyst::View::TT.

The "view" keyword tells the create script that you are creating a view.

The first argument "HTML" tells the script to name the View module "HTML.pm", which is a commonly used name for TT views. You can name it anything you want, such as "MyView.pm". If you have more than one view, be sure to set the default_view in Hello.pm (See Catalyst::View::TT for more details on setting this).

The final "TT" tells Catalyst the type of the view, with "TT" indicating that you want to use a Template Toolkit view.

If you look at lib/Hello/View/HTML.pm you will find that it only contains a config statement to set the TT extension to ".tt".

Now that the HTML.pm "View" exists, Catalyst will autodiscover it and be able to use it to display the view templates using the "process" method that it inherits from the Catalyst::View::TT class.

Template Toolkit is a very full-featured template facility, with excellent documentation at http://template-toolkit.org/, but since this is not a TT tutorial, we'll stick to only basic TT usage here (and explore some of the more common TT features in later chapters of the tutorial).

Create a root/hello.tt template file (put it in the root under the Hello directory that is the base of your application). Here is a simple sample:

<p>
This is a TT view template, called '[% template.name %]'.
</p>

[% and %] are markers for the TT parts of the template. Inside you can access Perl variables and classes, and use TT directives. In this case, we're using a special TT variable that defines the name of the template file (hello.tt). The rest of the template is normal HTML.

Change the hello method in lib/Hello/Controller/Root.pm to the following:

This time, instead of doing $c->response->body(), you are setting the value of the "template" hash key in the Catalyst "stash", an area for putting information to share with other parts of your application. The "template" key determines which template will be displayed at the end of the request cycle. Catalyst controllers have a default "end" action for all methods which causes the first (or default) view to be rendered (unless there's a $c->response->body() statement). So your template will be magically displayed at the end of your method.

After saving the file, the development server should automatically restart (again, the tutorial is written to assume that you are using the "-r" option -- manually restart it if you aren't), and look at http://localhost:3000/hello in your web browser again. You should see the template that you just created.

TIP: If you keep the server running with "-r" in a "background window," don't let that window get totally hidden... if you have a syntax error in your code, the debug server output will contain the error information.

Note: You will probably run into a variation of the "stash" statement above that looks like:

$c->stash->{template} = 'hello.tt';

Although this style is still relatively common, the approach we used previous is becoming more common because it allows you to set multiple stash variables in one line. For example:

Notice the "Local" attribute on the test method. This will cause the test action (now that we have assigned an "action type" to the method it appears as a "controller action" to Catalyst) to be executed on the "controller/method" URL, or, in this case, "site/test". We will see additional information on controller actions throughout the rest of the tutorial, but if you are curious take a look at "Actions" in Catalyst::Manual::Intro.

It's not actually necessary to set the template value as we do here. By default TT will attempt to render a template that follows the naming pattern "controller/method.tt", and we're following that pattern here. However, in other situations you will need to specify the template (such as if you've "forwarded" to the method, or if it doesn't follow the default naming convention).

We've also put the variable "username" into the stash, for use in the template.

Make a subdirectory "site" in the "root" directory.

$ mkdir root/site

Create a new template file in that directory named root/site/test.tt and include a line like:

<p>Hello, [% username %]!</p>

Once the server automatically restarts, notice in the server output that /site/test is listed in the Loaded Path actions. Go to http://localhost:3000/site/test in your browser and you should see your test.tt file displayed, including the name "John" that you set in the controller.